Doing science, a teacher (David Ost) argues, is like solving a crossword puzzle: where words interlock and cohere, one's confidence grows that the emerging answers are correct ... but when crossing words clash and contradict it's necessary to erase, backtrack, give up on otherwise-cherished hypotheses, try alternatives ... until things click and one suddenly sees the unanticipated meaning of subtle clues.

Nature is like that. Simple theories explain many phenomena, but as those theories are stretched they begin to break down. Newtonian physics works amazingly well --- until at high speeds or in microscopic realms or near strong gravitational fields it begins to crumble. The same goes for knowledge in every area.